Examples of an uplink channel of a communication system which is currently being discussed include a random access channel (RACH) for random access of a user equipment to a base station, and an uplink common channel (for example, HS-DPCCH) for transmitting channel quality indicator (CQI) and ACK/NACK information. Among them, the RACH is a random access channel which a user equipment performs downlink synchronization with a base station and can identify through base station information. Location, etc. of a corresponding channel can be identified from the base station information, and the RACH is a unique channel to which the user equipment can access in a state that the user equipment is not synchronized with the base station. If the user equipment transmits a signal to a corresponding base station through the RACH, the base station notifies the user equipment of correction information of uplink signal timing and various kinds of information for allowing the user equipment to connect with the base station. After the user equipment is connected with the base station through the RACH, communication can be performed using other unlink channel.
FIG. 1 and FIG. 2 illustrate examples of a procedure of connecting uplink communication between a user equipment and a base station.
The user equipment can acquire uplink and downlink synchronization with the base station and access a corresponding base station by accessing the RACH. Referring to FIG. 1, the user equipment is powered on and first connected with the base station. Referring to FIG. 2, if synchronization is dislocated or an uplink resource is requested (namely, a resource for uplink transmission data is requested) after the user equipment is initially synchronized with the base station, the user equipment accesses the base station.
First of all, in step (1) of FIG. 1 and FIG. 2, the user equipment transmits an access preamble to the base station, and also may transmit a message to the base station if necessary. Then, the base station identifies whether the corresponding user equipment has accessed the RACH for what purpose, and takes a corresponding action. In case of initial access as illustrated in FIG. 1, the base station allocates timing information and uplink data resources to the corresponding user equipment in steps (2) and (3), so that the user equipment can transmit uplink data as illustrated in step (4).
Meanwhile, step (1) of FIG. 2 illustrates an example of the reason why the user equipment accesses RACH is scheduling request (SR). In next step (2), the base station allocates timing information and resources for SR to the user equipment, and performs uplink data resource allocation (step (4)) in response to SR (step (3)) of the user equipment to allow the user equipment to perform uplink data transmission (step (5)).
In accessing the RACH, in case of FIG. 2 not initial access, different signals can be used depending on a signal sent to the RACH is synchronized with the base station.
FIG. 3 illustrates a structure of RACH signal used in a synchronized access and a non-synchronized access.
In case of the synchronized access, after performing synchronization with the base station, the user equipment accesses the RACH in a state that synchronization is maintained between the user equipment and the base station (synchronization is maintained through a downlink signal or control information such as CQ pilot transmitted to an uplink). In this case, the base station can easily identify signals included in the RACH. Since synchronization is maintained between the user equipment and the base station, the user equipment can use a longer sequence or transmit additional data as illustrated in an upper part of FIG. 3. On the other hand, in case of the non-synchronized access, if synchronization is not maintained between the user equipment and the base station for some reason when the user equipment accesses the base station, the user equipment should set a guard time as illustrated in a lower part of FIG. 3 in accessing the RACH. The guard time is set considering maximum round-trip delay of the user equipment which desires to receive a service within the base station.
The length of the RACH should depend on a size of a cell of the base station. Round-trip delay becomes great if the user equipment becomes far away from the base station. This means that the guard time set for the user equipment in the non-synchronized access becomes long. Also, if the size of the cell becomes great, path loss between the user equipment and the base station becomes great, whereby signals are required to be transmitted by spreading. This is illustrated in FIG. 4.
FIG. 4 illustrates the size of the cell and the length of the channel.
As illustrated in FIG. 4, the length of the channel, especially a time axis length of the RACH is set in proportional to the size of the cell in a place where a communication system will be actually installed. FIG. 4 illustrates an example of RACHs defined by three types depending on whether the size of the cell is divided into small size, medium size, or large size. Whether the cell is divided into what ranges may depend on conditions of the corresponding system.
How RACH signal is transmitted for different RACH lengths is divided into two types in the current 3GPP LTE. One type is to increase the length of the RACH signal to be provided in a minimum sized cell by simply repeating the RACH signal when the size of the cell becomes great. The other type is to use different sequences defined for channels of different lengths. If a signal of a minimum sized cell is repeatedly used, it is advantageous that the user equipment becomes simple correspondingly. On one hand, since a short sequence is repeatedly used to use a longer RACH, it is disadvantageous that an available random access sequence becomes short. On the other hand, if long sequences are respectively defined and used, it is advantageous that the base station can use a lager number of random access sequences with better detection performance. In this case, complexity of the user equipment increases.
FIG. 5 respectively illustrates a case where a single sequence is repeated to form a long channel signal and a case where different sequences are defined depending on each length.
Referring to FIG. 5, the case where a single sequence is repeated to form a long channel signal in a large cell is shown in an upper part while the case where different sequences are defined is shown in a lower part.
As described above, as the size of the cell increases, the length of the RACH, i.e., the number of subframes occupied by the RACH in a time region increases. Also, if the distance between the user equipment and the base station increases as the size of the cell increases, it is advantageous to increase a frequency width occupied by the RACH.
FIG. 6 illustrates that a frequency band occupied by the RACH increases depending on the size of the cell.
If the distance between the user equipment and the base station increases as the size of the cell increases, path loss of the RACH signal transmitted from the user equipment and delay spreading increase. Accordingly, if the size of the cell increases, the RACH has a longer time length as illustrated in FIG. 4 and a wider bandwidth to compensate for path loss. FIG. 6 illustrates that a bandwidth occupied by the RACH increases if the cell is divided into a small cell, a medium cell, and a large cell.
When sequences are defined and used as described above, requirements of the sequences are to increase success probability when user equipments (UE) in the boundary of the cell access the base station. However, among user equipments within the large cell, not only user equipments having the same distance from the base station as the size of the cell but also user equipments close to the base station exist. Accordingly, sequences according to requirements of the user equipments located in the boundary of the cell are generated based on these user equipments only in a design method which does not consider that success probability inside the cell is different from success probability in the boundary of the cell. If the generated sequences are used as illustrated in FIG. 4 and FIG. 5, a problem occurs in that the user equipments close to the base station are forced to use sequences of excessive requirements. A concept of “segmented access” in which different requirements are required depending on the location of the user equipments within the large cell is not recognized conventionally.
Meanwhile, how sequences applied to the RACH for data transmission to the base station in the aforementioned synchronized access and non-synchronized access are divided and allocated to the user equipments will be described below.
If the number of a total of sequences to be actually used in the RACH is determined, how to use the sequences in each cell should be determined. In the current 3GPP LTE (hereinafter, LTE) system, 215=32768 sequences are required, and a random access selected by one user equipment should indicate 6 bits. Accordingly, 64 sequence groups are set. In this case, 512 random access groups are required. In this way, a total number of available sequences should be allocated to each cell, and a method of reusing the sequences is limited depending on whether a sequence based system is a synchronized network or a non-synchronized network.
In the current LTE system, it is difficult to increase reuse of the RACH by differently allocating the sequences on a time-frequency region. Accordingly, the technology of increasing efficient reuse of the sequence in sequence allocation is required. The present invention intends to suggest a method of solving the aforementioned problem considering the requirements which depend on the distance the user equipment and the base station within the cell like the aforementioned segmented access.